The field of the disclosure relates generally to gas turbine engines and, more particularly, to acoustic liners for gas turbine engine components.
Aircraft engine noise is a significant problem in high population areas and noise-controlled environments. The noise is generally composed of contributions from various source mechanisms in the aircraft, with fan noise typically being a dominant component at take-off and landing. Fan noise is generated at the fan of the aircraft engine, propagates through the engine intake duct, and is then radiated to the outside environment. Acoustic liners are known to be applied on the internal walls of the engine's casing to attenuate the fan noise propagating through the engine ducts. Typical acoustic liners for engines are either a single degree of freedom (SDOF) liner, or a two degree of freedom (2DOF) liner, sometimes referred to as a double degree of freedom (DDOF) liner.
SDOF liners are formed of a porous facing sheet backed by a single layer of cellular separator such as honeycomb cells, which itself is backed by a solid backing plate that is substantially impervious to higher frequency noise transmission. 2DOF liners, on the other hand, are formed of two cellular layers between the porous facing sheet and the solid backing plate, with the two cellular layers separated by a porous septum sheet. The acoustic performance of both SDOF and 2DOF liners is strongly dependent on the depth of the cells in each honeycomb layer, where the cell depth controls the internal volume of the cell that is available for acoustic resonance. The additional layer of the 2DOF liner allows noise suppression of at least one other main frequency than the SDOF liner. However, the additional layer of the 2DOF liner significantly increases the weight of and cost to produce the liner.
At least some known SDOF honeycomb acoustic liners attempt to achieve the multiple frequency advantages of the 2DOF liner in an SDOF construction by forming individual cells within the core layer to have variable depths from the perforate facing sheet, thereby creating different resonant cavity volumes within the same SDOF layer. However, this variable depth construction requires a thicker core layer to accommodate the depth of longer individual cells that correspond to larger cavity volumes. Additionally, because some of the variable depth cells have shorter lengths, there is left a significant amount of solid material between the bottom of the shorter cell and the backing plate, which also increases the overall weight of the core layer.